1. Technical Field
The present invention relates to a lens having a zooming function, and an imaging apparatus using such a zoom lens as an imaging optical system.
2. Description of the Related Art
For video cameras and TV cameras, there are requests over various fields from users. Among them, high image quality and a small size/light weight have been constantly requested from users and occupy large weights of the requests from the users. Accordingly, implementation of both high performance and a small size/light weight is requested also for zoom lenses that are used as imaging optical systems of such cameras.
Particularly, in video cameras for monitoring, such as in monitoring cameras, there are cases where imaging requiring sensitivity of up to a near-infrared region having a wavelength of about 900 nm or less is performed. For example, an operation is performed such that a precise color image is acquired using only visible light by cutting off near-infrared light at day time when the amount of light is sufficient, the amount of light is earned by allowing all the light of the visible region to the near-infrared region to be transmitted at the time of bad weather, at dusk, or at dawn when the amount of light tends to decrease, and infrared light having a wavelength of about 850 nm is transmitted for illumination by cutting off visible light at night time.
Accordingly, for imaging lenses, such as zoom lenses used as imaging optical systems, the correction of chromatic aberration of not only the visible region but also up to the near-infrared region is required. The reason for this is that, in a case where correction of the chromatic aberration of up to the near-infrared region described above is not performed, focusing needs to be performed again at the time of switching between visible light and near-infrared light, or sufficient resolving power cannot be acquired in a case where all the light of the visible region to the near-infrared region is transmitted so as to be used.
In addition, it is preferable that the zoom ratio should be as high as can be. For a relatively-long distance monitoring use, a relatively-small zoom lens having a zoom ratio of over 25 times and being close to 30 times is supported by the market as one option.
As zoom lenses that are appropriate to such monitoring video cameras, several types may be considered. Among them, a zoom lens that is suitable for a high zoom ratio is formed by sequentially arranging a first lens group having a positive focal length such as positive refractive power, a second lens group having a negative focal length such as negative refractive power, a third lens group having a negative focal length such as negative refractive power, and a fourth lens group having a positive focal length such as positive refractive power from the object side to the image side. There are zoom lenses in which the second lens group and the third lens group are moved in accordance with a zooming operation such that the second lens group is positioned on the most object side at the wide angle end, and the third lens group is positioned on the most image side at the telephoto end when the zooming operation is performed.
In a zoom lens of this type, in order to sufficiently correct the chromatic aberration, generally, a special low-dispersion glass material that is represented by OHARA S-FPL51 or OHARA S-FPL53 (both are glass materials manufactured by Ohara Inc.) is used for the first lens group in which the marginal ray height on the axis increases on the telescopic side and the fourth lens group in which the marginal ray height on the axis increases on the wide angle side. In addition, it has been proposed that a diffraction optical element is disposed in the first lens group, and the chromatic aberration is corrected using negative dispersion of the diffraction optical element.
While various zoom lenses are available, there is no zoom lens in which the above-described needs are sufficiently satisfied at the same time.